One of the greatest challenges gem dealers and gemologists
face today is being able to accurately determine if a stone has been heat treated.
While a 100% reliable answer to that question is a job for a major gem lab, there
is a simple and inexpensive tool that can often give an important indication.

So what is this miracle tool? We speak
of the lowly ultraviolet light.

It wasn’t long ago that ultraviolet
(UV) fluorescence was considered the poor stepchild of the gem lab, a pint-sized
pea shooter when compared with the high-caliber cannons available in modern labs.
But with the rising importance of treatment detection, the humble UV lamp is making
a comeback.

Reactive

Many heat-treated rubies and sapphires will display chalky short-wave (SW) fluorescence.
This reaction is practically never found in untreated corundums and was first noted
by Robert Crowningshield (1966, 1970). It is actually the colorless portions of
the stone that fluoresce (a reaction similar to Verneuil synthetic sapphires).
Since colorless areas follow the original crystal’s growth structure, the
fluorescence will follow the same pattern as the gem’s color zoning. In addition, other trace elements in corundum may produce fluorescent reactions, from the well-known red glow of ruby to other reactions that are still not completely understood. Many will be illustrated below, both known and unknown.

Figure 1. Tufted love
When a sapphire is subjected to high-temperature heat treatment, a chalky
blue to blue-green SW fluorescence is often created. As seen above, this reaction is confined
to certain zones in the gem. These “tufted” fluorescent zones follow the crystallographic structure
of the gem. Photo: Richard W. Hughes; Nikon D70

Figure 2. Ring around the collar
Flipping over the same sapphire from Figure 1 reveals a distinct bluish
(‘chalky’)
fluorescent ring, corresponding to the colorless portions of the gem when viewed in immersion.
When seen, this strong chalky blue SW fluorescence is an extremely strong indication that
the gem has been subjected to high-temperature heat treatment. Photo: Richard W. Hughes; Nikon D70

Figure 4.
Another
blue sapphire showing chalky fluorescence corresponding to the colorless portions of the gem.
When seen, this strong chalky blue to green SW fluorescence is an extremely strong indication
that the gem has been subjected to high-temperature heat treatment. Note that this fluorescence
often appears in patterns that resemble the graining of wood. Photo: Richard W. Hughes;
Nikon D200

Figure 5. Fluorescent fillers in emerald
UV fluorescence can also help identify treatments in emerald. Emeralds are typically enhanced
by filling their fissures with oils/resins. Some of these fluoresce. In the photo above,
an emerald is exposed to long-wave UV light. The filler in the fissures is clearly identified
by its bluish fluorescence. Note that the body color of the stone appears much different
under UV light. Photo: Richard W. Hughes; Nikon D70

Show and tell

So how does one go about checking for this reaction? The first step is to obtain
a combination LW/SW lamp. You will also need a pair of protective glasses (SW light
can burn your eyes with prolonged exposure). A viewing cabinet is also a plus.
Finally, you will need a small lens to magnify the stone. Figures 6 –8 show
one setup for both viewing and photographing fluorescence.

When observing fluorescence, the idea
is to hold the stone with tweezers and bring it as close as possible to the lamp
and view it under magnification. Examine the stone from all angles; many times the
key chalky areas are confined to tiny portions of the stone.

One of the authors (Hughes, 1997) has
suggested a lens be incorporated as an integral part of the viewing cabinet, but
sadly instrument manufacturers have yet to produce such a unit.

Figure 6.
Setup for close-up examination of UV fluorescence. A small piece of blutac or clay inserted
between the UV lamp and the viewing cabinet creates a small gap. This allows the stone
to be viewed while positioned extremely close to the lamp, greatly increasing the ability
to catch weak reactions. A lens is positioned to magnify the stone during viewing. The
masking tape keeps the lamp from tipping off the cabinet. Special UV protection glasses
such as those in front of the cabinet should be worn to protect the eyes from harmful SW
radiation. Photo: Richard W. Hughes; Nikon D70

Figure 7. Photographing fluorescence
Some photos in this article were taken using the following crude set-up. A stoneholder was
taped to a gooseneck arm to hold the stone in place. Photos were shot with a tripod-mounted
Nikon D70 digital camera with a Nikon 60mm 2.8 macro lens. Also finding sometime use were
two Nikon screw-on close-up lenses, to further increase magnification. Exposures were made
in manual mode with a wireless shutter release, with all ambient (room) lights turned off.
Exposures ranged from a few seconds to over half a minute.

Later, the setup was refined to a tripod-mounted Nikon D200 and the aforementioned
60mm 2.8 macro lens. In a darkened room, with the UV lamp hand-held, some spectacular shots
were obtained. Photo: Richard Hughes

Figure 8. Another view
Another view of the photo setup. Photo: Richard Hughes

Caveats

This test does require a bit of knowledge. If a ruby or sapphire shows a chalky fluorescence
in SW, it is probably heat treated. If it is inert, that does not mean
it’s unheated. Also be careful that the stone is clean. Soap and other chemicals
can also produce chalky fluorescence. And while this test is a tool that can be
extremely useful, it is not a substitute for a complete gemological examination
in a fully-equipped laboratory. Finally, keep the exposure times of corundum to
SW fluorescence to a minimum. SW irradiation does create a yellow color center
that can alter the color of the gem; even five minutes exposure can do this (see
Figure 9). While this color fades with prolonged exposure to daylight, it can turn
a blue stone more greenish (not good if it’s your stone and you’re
trying to sell it).

Figure 9.
At left is a blue sapphire
in a ring; at right, the same stone following a few minutes irradiation by SW UV. This
yellow color will fade with exposure to sunlight, but illustrates how one should not expose
corundums for prolonged periods to SW UV. Photos: Richard Hughes; film

One further caveat concerns a type of chalky green SW fluorescence sometimes seen in natural, untreated blue sapphires (particularly those from Madagascar). The fluorescence tends to be weak, and extremely superficial, being limited to thin layers at the surface. In addition, the fluorescent patches tend to have sharper boundaries than the reaction in heated stones (Figures 10–12).

Figure 10.
Occasionally we see a chalky
green SW fluorescence in untreated natural sapphires, particularly those from Madagascar.
This fluorescence tends to be restricted to a thin layer at the surface of the stone and
has sharp boundaries, as shown above. The small black triangular area is simply a non-fluorescent
zone. Photo: Richard W. Hughes; Nikon D200

Figure 12.
Using fiber-optic illumination
with the microscope, the fluorescent patch in the stone from Figure 11 is revealed as a clear
area without texture clouds. Photo: Richard W. Hughes; Nikon D200

Breaking down fluorescence

In its most basic sense, fluorescence is the emission of visible energy of a longer wavelength
when bombarded by energy of a shorter wavelength.
The stimulating energy may be x-rays (x-ray fluorescence), ultraviolet light (UV
fluorescence) or even visible light. Ruby provides an excellent example of the latter.

When a ruby is put into daylight, certain electrons
are excited to higher orbitals, producing absorption of the corresponding wavelengths.
But instead of falling straight back to the ground state, the electrons fall in steps.
In most cases, the release of energy from each of those steps is in the form of phonons
to the crystal lattice (vibrational heat), and thus invisible to the human eye. But
in the case of ruby, some emissions fall into the red (at 692.8 and 694.2 nm). This
is what makes ruby so special; not only does it possess a red body color, but that
red body color is supercharged by red fluorescence. This is what led the ancients
to believe ruby had a fire burning inside.

UV fluorescence can be an extremely sensitive
indicator not only of trace impurities, but also the conditions under which the gem
formed. Indeed, it is not unusual for fluorescence to be easily seen from strongly-fluorescing
ions at concentrations in the range of 0.01 parts-per-million (ppm). For lay people,
that’s an itsy-bitsy amount, completely beyond the detection limits of all
but the most sophisticated and expensive analytical equipment.

Figure 13.
One
of the remaining corundum mysteries is the cause of the "apricot" orange fluorescence
seen in many sapphires of both blue and yellow color, particularly those from Sri Lanka and
Madagascar. This fluorescence may be seen in both LW and SW, with LW always being stronger,
and is unaffected by heat treatment. The above stone is an untreated Madagascar blue sapphire
in LW, the same stone as shown in Figures 11 and 12. Note that the culet area, which contains
the heaviest concentration of blue color, is inert. Photo: Richard W. Hughes; Nikon
D200

Speaking of sapphire

While the red fluorescence of ruby is detailed in many gemological texts (c.f. Hughes,
1997), the cause of the chalky fluorescence has not been covered. Let’s take
a look at it.

Sapphire generally shows no fluorescence
to visible light. But that changes if we expose it to short-wave UV. This is most
clearly seen in synthetic colorless sapphire, which displays a bluish white (‘chalky’)
emission in the range of 410–420 nm.

Synthetic sapphire

This blue fluorescence in synthetic sapphire has been observed at least since 1948.
While it has been generally ignored in the gemological literature, it has been
the subject of numerous scientific papers (c.f. Evans, 1994).

Evans surmised after reviewing the data that the
410–420 nm fluorescent peak was due to Ti4+ charge-transfer
transition. That was later confirmed by Wong, et al. (1995a and 1995b). Isolated Ti4+ ions, or Ti–Al vacancy pairs
produce this fluorescence.

The Ti4+ charge-transfer transition in
corundum is so strong and the efficiency so high that the fluorescence is easily
observed by eye at even just 1 ppm Ti4+. Most of the synthetic sapphire
in the market contains at least one ppm of Ti4+ from the Al2O3 starting
material, if not more, and thus fluoresces. The fluorescence peaks at about 415 nm
at very low Ti4+ concentrations, but as the concentration increases, the
fluorescent band broadens and the peak shifts to as high as 460 or 480 nm, making
the fluorescence appear more greenish-blue or whitish-blue.

Why this chalky fluorescence occurs relates
to the growth temperature and Ti4+ concentrations relative to other impurities.
In synthetic corundums, the high growth temperatures and high Ti4+ concentrations
produce the chalky fluorescence. In certain heat-treated sapphires with low Fe levels
(such as those from Sri Lanka), high-temperature heat treatment creates similar conditions
to the synthetic. Thus the chalky fluorescence.

Natural sapphire

But what about natural, untreated sapphires? Why don’t they fluoresce blue
or bluish white? The reason relates to growth temperatures and time. Natural sapphires
grow at much lower temperatures, so Ti4+ is much less likely to pair up
with Al vacancies.

These lower temperatures also allow easier
pairing of Ti4+ with other ions (usually Fe2+ or Mg2+)
that prevent fluorescence. Another damper is the presence of Fe3+, which
also kills fluorescence. And finally, as the crystal sits in the ground for millions
of years, diffusion slowly takes place, allowing the Ti4+ to slowly pair
up with other ions, thus killing the fluorescence.

Heat-treated sapphire

Why then, do some heat-treated blue sapphires fluoresce chalky blue to green or white,
and what causes the difference in appearance?

When blue (or geuda)
sapphires are found in nature, they usually contain exsolved rutile. Titanium is
concentrated in these rutile micro-crystals. When the stone is heat treated, the
rutile dissolves into the corundum by diffusion, but because diffusion is slow, the
local concentration of Ti4+ can be quite high. In the high concentration
regions the Ti4+ concentration will exceed the local charge compensators
(Fe2+ or Mg2+) and thus free Ti4+ ions will form.
In addition, the dissolution of rutile will locally force the creation of some aluminum
vacancies and some of the Ti4+–Al vacancy clusters will form. These
types will fluoresce and thus some heat-treated sapphire will fluoresce somewhat
like synthetic Ti-bearing sapphire. Because the original distribution of rutile (and
iron in solution) occurred in zones, the distribution of the fluorescence will reflect
that zoning. The fluorescence will be most intense where Fe is lowest and Ti4+ is
highest, i.e. in areas of minimal color. The high iron-content basaltic sapphire
(such as that from Australia, Thailand, etc.) will not fluoresce after heat treatment,
as the iron concentration is much higher than the Ti4+ concentration everywhere.

All of the above is summarized in this table:

Material

Impurity
Levels

Growth
Temperature

Growth
Speed

Chalky
SW UV
Fluorescence

Synthetic
sapphire

Low

High

Fast

Chalky

Natural
sapphire

Various

Low

Slow

Inert

Heat-treated
sapphire (low Fe type)

High
Ti relative to Fe

Initially
low; high during treatment

Slow;
fast during treatment

Chalky
in zones

Heat-treated
sapphire (Fe-rich type)

Low
Ti relative to Fe

Initially
low; high during treatment

Slow;
fast during treatment

Inert

Superficial

The appearance of chalky fluorescence in a corundum depends
strongly on both the Ti4+ and Fe3+ concentrations. Considering
Ti4+ first, it is important to note that the charge-transfer absorption
in the UV per ion is extremely high. If we look at the fluorescence of a piece of synthetic
sapphire with several ppm of Ti4+, it seems to glow blue throughout the
volume. This is because the total Ti4+ charge-transfer absorption is low
enough that the UV photons can penetrate into the bulk of the sample. When the Ti4+ concentration
is higher, the fluorescence seems to be coming from a thick layer near the surface
because that is as far as the UV photons can penetrate. At high Ti4+ concentrations,
only a thin surface layer is penetrated by the UV and the fluorescence appears as a
chalky surface layer (see Figures 15 & 16). The charge transfer absorption of Fe3+ is
also very high. Thus iron will contribute to limiting the penetration of UV into the
sample also. Thus the very different appearance of the fluorescence of some synthetic
sapphire and some heat-treated natural sapphire is not a different phenomenon, just
a difference in impurity concentration.

One of the authors (JLE) has used a Schott
BG-12 filter to heighten the superficial chalky fluorescence often seen in heat-treated
ruby. This filter eliminates the red fluorescence and transmits the Ti4+ blue
fluorescence (Figure 17).

Figure
17. It’s easy being green
In the same stone from Figure 16, a green Schott BG-12 filter is placed over the gem. It
removes the red fluorescence, thus making the chalky blue areas far easier to see. SW UV. Photo:
Richard W. Hughes; Nikon D70

The traditional gemological versus the
laser junkie

While laser junkies focus strongly on the subject of fluorescence of ions in crystals,
it is an orphaned topic in gemology. Let’s look at the different approaches.

With gemology, fluorescence is typically
only visually observed with either LW or SW UV radiation, with the results recorded
in terms of just brightness, color, and the presence or absence of phosphorescence.

In the study of ions in crystals, the
parameters measured are more extensive. Typically the spectral distribution of the
fluorescence is measured, as well as the spectral distribution of the light that
can excite that fluorescence (excitation spectrum). In addition, the temporal decay
parameters of the fluorescence are measured using a short pulse light source. Sometimes
the decay curve is a single exponential indicating a single site or a single ion.
Other times the decay curve is a combination of two or more exponentials indicating
multiple sites or multiple ions. All of these parameters are often measured as a
function of temperature.

Figure 18. The myth of purity
Crystals arise not from an ideal source, but spread out from a mixed broth. As they
grow, the composition of that nectar changes because each influences the other. Diffusion is always a two-way street. The above photo is a map-like microcosm of this concept. Compositional areas lie
tightly bounded in places, while blurred in others. The notion that either terrestrial
or biologic creations might be "pure" is a myth. All are products of the
past, all are affected by the present, all will be affected by the environment in which they reside, while simultaneously affecting that environment themselves.
Each of these conditions is unique to the individual, as the above image shows. Thus no two will ever be alike. Heat-treated
blue sapphire in SW UV. Photo: Richard W. Hughes; Nikon D200

Summing up

With UV fluorescence, we have something all too rare in gemology today: an inexpensive
test that is as sensitive as even bomb-science level analytical equipment.

Now what does this mean for a gem dealer?
With a small UV lamp, one can quickly check potential purchases. Any stones that
show a chalky SW fluorescence are most likely heat treated. Total equipment outlay?
The lamp alone costs less than $300. Heh, heh, heh, we can already see you smiling.

And for the laboratory gemologist? This
technique has been under-appreciated in the gemological community. Fluorescence might
provide an avenue to determine if some sapphires have received heat treatment at
temperatures lower than those normally used for geuda. But this will require adopting some of the techniques and instrumentation
of the laser junkie. While expenditures for such instrumentation are hard to justify
without a guarantee of just what may be learned, the increasing need to stay abreast
of gemstone treatments requires an expansion of our array of techniques. Sophisticated
fluorescence instrumentation is far less expensive than SIMS, LA-ICP-MS or LIBS analysis. Perhaps not quite so sexy (nor nearly as expensive), but when it comes to utility, this still looks like a pretty decent dance partner.

Acknowledgements

RWH wishes to thank John I. Koivula for his encouragement
and suggestions during the writing of this article.

About the authors

Richard Hughes is the author of the classic Ruby & Sapphire and
over 100 articles on various aspects of gemology. His writings can be found on
his personal web site, www.ruby-sapphire.com.
Dr. John Emmett is one of the world’s foremost
authorities on the heat treatment, physics, chemistry and crystallography of corundum.
He is a former associate director of Lawrence Livermore National Laboratory and a
co-founder of Crystal Chemistry, which is involved with heat treatment of gemstones.

Notes

This article came about following RWH's plunge back into
serious gemology in January 2005, when he joined the AGTA
GTC. While checking the SW fluorescence of a heated sapphire, he decided to call
JLE to inquire about the cause of the chalky fluorescence in heated and synthetic
sapphires. "Interesting that you should ask," Emmett replied. "I've
been doing much thinking about that same subject of late." And so it was that
RWH and JLE began sharing thoughts on this subject.

Penned
in bits and pieces in the first half of 2005, an edited version appeared in The
Guide (Sept.–Oct. 2005, Vol. 24, Issue 5, Part 1, pp. 1,
4–7. Pieces also appeared as part of the AGTA
GTC's regular Laboratory Updates.

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